1. Field of the Invention
The invention relates to a method for heating an electrical heating element, in particular a glow plug for an internal combustion engine, from an initial temperature to an operating temperature.
2. Description of Related Art
In order to rapidly heat an electrical heating element, such as a glow plug of a steel or ceramic design, or another type of heating element, such as a heater or heating flange, to an operating temperature, the heating element is usually operated for a limited time at a voltage higher than the operating or set voltage of the heating element. In this case, the set voltage of a heating element is the voltage at which the heating element has reached and maintains its operating parameters, in particular its desired or required surface temperature in a steady state. As a rule, the emitted thermal power of the heating element is equal to the consumed electrical power in this steady state.
Therefore, the heating process differs from the steady operating state in that a power that is higher than the power requirement in the steady state is supplied to the heating element for a limited time. This is necessary, since the heating element has mass which forms a thermal capacitance. This thermal capacitance must be taken into account during the heating process, and hence the specific amount of energy supplied to the heating element during the heating process. During the heating process, this provides an additional amount of energy increases the temperature of the heating element form its initial temperature to the operating temperature.
However, if the operating temperature determined by the design of the heating element is very high, and if there is only a very slight temperature difference relative to the melting point of the material of the heating element, e.g., the operating temperature for a glow plug is 1200xc2x0 C. and the melting point is 1400xc2x0 C., there is a danger that too much energy will be supplied to the heating element during the heating process, thereby causing the heating element to heat up to a temperature exceeding the operating temperature. If the melting point of the heating element is reached in the process, the heating element is destroyed. However, damage is usually already done to the heating element when the temperature of the heating element comes within its melting point range, i.e., when the reached heating element temperature is just 100xc2x0 C., for example, under the melting point of the heating element.
The problem described above, namely that too much energy is supplied to the heating element during the heating process, can be caused by heating the heating element without taking into consideration the initial temperature of the heating element at the beginning of the heating process, i.e., the initial temperature that is higher than normal but under the operating temperature that may be a result of a recently operated heating element or an already high ambient or engine temperature. Since the energy required for heating depends on the temperature difference between the operating temperature and the initial temperature of the heating element, too much energy is then supplied to the heating element without considering the importance of the heating element initial temperature such that the heating element is brought to a temperature exceeding the operating temperature.
Therefore, a primary object of the present invention is to provide a method for preventing a heating element from overheating, or from heating to a temperature higher than its intended operating temperature, so as to avoid damaging or even destroying the heating element.
This object is achieved according to the invention by determining the initial temperature of the heating element and supplying electrical energy to the heating element for heating purposes at a level dependent on the determined initial temperature.
Therefore, in the method according to the invention, the initial temperature of the heating element is determined so that the temperature difference, and hence the electrical energy, required to bring the heating element from the determined initial temperature to the operating temperature can be determined using the given operating temperature. This makes it possible to avoid overheating and damaging the heating element.
The present invention will be explained in greater detail with a preferred example of a method for heating a heating element, e.g., a glow plug, in conjunction with the accompanying figures of the Drawings.